Parthenogenesis and Polyploidy in Mammalian Development

Parthenogenesis and Polyploidy in Mammalian Development

Parthenogenesis and Polyploidy in Mammalian Development

Parthenogenesis and Polyploidy in Mammalian Development

Excerpt

This book is mainly about the significance in mammals of two remarkable variants to normal development, known for some time in lower animals: parthenogenesis, in which an embryo develops from an egg which has not been fertilized by a spermatozoon; and heteroploidy, in which whole sets of chromosomes may be duplicated (polyploidy), missing, or incomplete. In several ways, these two phenomena are related.

About twenty years ago, Pincus and his associates demonstrated parthenogenetic development of the rabbit egg, and even claimed that such eggs could occasionally come to term as viable parthenogenetic young. At the same time, it was reported that abnormal chromosome numbers had been recognized in early embryos. During the last decade, there has been renewed interest in these phenomena, and also, parallel to and somewhat apart from these studies, the possible role of heteroploidy in the evolution of mammals has been re-examined. In the last six years it has been claimed, for instance, that a heteroploid pig and rabbits have been born, and that at least one rodent species and even man are heteroploids of evolutionary origin.

The primary intention of the present book is to take stock of our knowledge of these and related phenomena in mammals, with the non-mammalian background outlined for comparative purposes. The facts have frequently to be integrated at several different levels, notably the cytological, genetical, cytogenetical and embryological. The subject-matter lies at the meeting- point of these disciplines. There seems to be no single word to describe such an approach. 'Genetics', in Bateson's original sense of 'the physiology of descent', would have been suitable, but this word is now generally understood in somewhat narrower senses. 'Developmental biology' is perhaps the nearest descriptive title.

In general terms, it may be said that the study of these special modes of existence in mammals is at a half-way stage. In the mammalian embryo, most of the types of abnormal . . .

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